Fix TypeIs narrowing before isinstance

Author: nkgotcode

mypy/checker.py

@@ -6591,11 +6591,23 @@ def find_isinstance_check_helper(
                 if len(node.args) != 2:  # the error will be reported elsewhere
                     return {}, {}
                 if literal(expr) == LITERAL_TYPE:
+                    original_type = self.lookup_type(expr)
+                    current_type = self.expr_checker.narrow_type_from_binder(
+                        expr, original_type
+                    )
+                    yes_type, no_type = self.conditional_types_with_intersection(
+                        current_type, self.get_isinstance_type(node.args[1]), expr
+                    )
+                    if (
+                        self.binder.get(expr) is not None
+                        and yes_type is not None
+                        and is_subtype(current_type, yes_type, ignore_promotions=True)
+                    ):
+                        yes_type = current_type
                     return conditional_types_to_typemaps(
                         expr,
-                        *self.conditional_types_with_intersection(
-                            self.lookup_type(expr), self.get_isinstance_type(node.args[1]), expr
-                        ),
+                        yes_type,
+                        no_type,
                     )
             elif refers_to_fullname(node.callee, "builtins.issubclass"):
                 if len(node.args) != 2:  # the error will be reported elsewhere

test-data/unit/check-typeis.test

@@ -26,6 +26,25 @@ def main(a: Union[Point, Line, int]) -> None:
 
 [builtins fixtures/tuple.pyi]
 
+[case testTypeIsAndIsinstanceWithGenericAlias]
+from typing import Any, Generic, TypeVar
+from typing_extensions import TypeAlias, TypeIs
+
+T = TypeVar("T")
+class Slice(Generic[T]): pass
+
+SliceInt: TypeAlias = Slice[int | None]
+SliceStr: TypeAlias = Slice[str | None]
+
+def is_slice_int(obj: Any) -> TypeIs[SliceInt]: pass
+
+def main(obj: SliceInt | SliceStr) -> None:
+    if is_slice_int(obj):
+        pass
+    elif isinstance(obj, Slice):
+        reveal_type(obj)  # N: Revealed type is "__main__.Slice[builtins.str | None]"
+[builtins fixtures/isinstance.pyi]
+
 [case testTypeIsTypeArgsNone]
 from typing_extensions import TypeIs
 def foo(a: object) -> TypeIs:  # E: TypeIs must have exactly one type argument